The invention relates to the field of electrophoretic separations of macromolecules and in particular, to the automation of two-dimensional electrophoretic separations used in the analysis of proteins. Such two-dimensional procedures typically involve sequential separations by isoelectric focusing (IEF) and SDS slab gel electrophoresis, and an automated 2-D method thus involves manufacture and use of gel media for both isoelectric focusing and SDS electrophoresis, together with means for protein detection and quantitation. Two-dimensional electrophoresis technology forms the basis of the expanding field of proteomics, and hence automation of the procedure is a critical requirement for scale-up of efforts to build proteome databases comprising all the proteins of complex organisms such as man. To date no successful automation efforts have been reported, despite the use of bench-scale 2-D electrophoresis in more than 5,000 scientific publications.
The publications and other materials used herein to illuminate the background of the invention and in particular, cases to provide additional details respecting the practice, are incorporated herein by reference, and for convenience are referenced in the following text and respectively grouped in the appended List of References. Elements of the invention are disclosed in our Disclosure Documents 393753, 393754 and 412899.
Isoelectric Focusing (IEF)
A protein is a macromolecule composed of a chain of amino acids. Of the 20 amino acids found in typical proteins, four (aspartic and glutamic acids, cysteine and tyrosine) carry a negative charge and three (lysine, arginine and histidine) a positive charge, in some pH range. A specific protein, defined by its specific sequence of amino acids, is thus likely to incorporate a number of charged groups along its length. The magnitude of the charge contributed by each amino acid is governed by the prevailing pH of the surrounding solution, and can vary from a minimum of 0 to a maximum of 1 charge (positive or negative depending on the amino acid), according to a titration curve relating charge and pH according to the pK of the amino acid in question. Under denaturing conditions in which all of the amino acids are exposed, the total charge of the protein molecule is given approximately by the sum of the charges of its component amino acids, all at the prevailing solution pH.
Two proteins having different ratios of charged, or titrating, amino acids can be separated by virtue of their different net charges at some pH. Under the influence of an applied electric field, a more highly charged protein will move faster than a less highly charged protein of similar size and shape. If the proteins are made to move from a sample zone through a non-convecting medium (typically a gel such as polyacrylamide), an electrophoretic separation will result.
If, in the course of migrating under an applied electric field, a protein enters a region whose pH has that value at which the protein's net charge is zero (the isoelectric pH), it will cease to migrate relative to the medium. Further, if the migration occurs through a monotonic pH gradient, the protein will "focus" at this isoelectric pH value. If it moves toward more acidic pH values, the protein will become more positively charged, and a properly-oriented electric field will propel the protein back towards the isoelectric point. Likewise, if the protein moves towards more basic pH values, it will become more negatively charged, and the same field will push it back toward the isoelectric point. This separation process, called isoelectric focusing, can resolve two proteins differing by less than a single charged amino acid among hundreds in the respective sequences.
A key requirement for an isoelectric focusing procedure is the formation of an appropriate spatial pH gradient. This can be achieved either dynamically, by including a heterogeneous mixture of charged molecules (ampholytes) into an initially homogeneous separation medium, or statically, by incorporating a spatial gradient of titrating groups into the gel matrix through which the migration will occur. The former represents classical ampholyte-based isoelectric focusing, and the latter the more recently developed immobilized pH gradient (IPG) isoelectric focusing technique. The IPG approach has the advantage that the pH gradient is fixed in the gel, while the ampholyte-based approach is susceptible to positional drift as the ampholyte molecules move in the applied electric field. The best current methodology combines the two approaches to provide a system where the pH gradient is spatially fixed but small amounts of ampholytes are present to decrease the adsorption of proteins onto the charged gel matrix of the IPG.
It is current practice to create IPG gels in a thin planar configuration bonded to an inert substrate, typically a sheet of Mylar plastic which has been treated so as to chemically bond to an acrylamide gel (e.g., Gelbond.RTM. PAG film, FMC Corporation). The IPG gel is typically formed as a rectangular plate 0.5 mm thick, 10 to 30 cm long (in the direction of separation) and about 10 cm wide. Multiple samples can be applied to such a gel in parallel lanes, with the attendant problem of diffusion of proteins between lanes producing cross contamination. In the case where it is important that all applied protein in a given lane is recovered in that lane (as is typically the case in 2-D electrophoresis), it has proven necessary to split the gel into narrow strips (typically 3 mm wide), each of which can then be run as a separate gel. Since the protein of a sample is then confined to the volume of the gel represented by the single strip, it will all be recovered in that strip. Such strips (Immobiline DryStrips) are produced commercially by Pharmacia Biotech.
While the narrow strip format solves the problem of containing samples within a recoverable, non-cross-contaminating region, there remain substantial problems associated with the introduction of sample proteins into the gel. Since protein-containing samples are typically prepared in a liquid form, the proteins they contain must migrate, under the influence of the electric field, from a liquid-holding region into the IPG gel in order to undergo separation. This is typically achieved by lightly pressing an open-bottomed rectangular frame against the planar gel surface so that the gel forms the bottom of an open-topped but otherwise liquid-tight vessel (the sample well). The sample is then deposited in this well in contact with the gel surface forming the bottom of the well. Since all of the sample protein must pass through a small area on the surface of the gel (the well bottom) in order to reach the gel interior, the local concentration of protein at the entry point can become very high, leading to protein precipitation. The sample entry area is typically smaller than the gel surface forming the well bottom because the protein migrates into the gel under the influence of an electric field which directs most of it to one edge of the well bottom, tending to produce protein precipitation. The major source of precipitation, however, is provided by the charged groups introduced into the gel matrix to form the pH gradient in IPG gels: these groups can interact with charges on the proteins (most of which are not at their isoelectric points at the position of the application point and hence have non-zero net charges) to bind precipitates to the gel. It is common experience that separations of the same protein mixture on a series of apparently identical IPG gels can yield very different quantitative recoveries of different proteins at their respective isoelectric points, indicating that the precipitation phenomenon may vary from gel to gel in unpredictable ways, thereby frustrating the general use of IPG gels for quantitative protein separations.
Recently, methods have been introduced in which the IPG strip is re-swollen, from the dry state, in a solution containing sample proteins, with the intention that the sample proteins completely permeate the gel at the start of the run.
Isoelectric focusing separation of proteins in an immobilized pH gradient (IPG) is extensively described in the art. The concept of the IPG is disclosed in U.S. Pat. No. 4,130,470 and is further described in numerous publications. The IPG gel strips manufactured are generally of simple planar shape.
A series of disclosures have dealt with various configurations of cavities ("sample wells") used for the application of macromolecular-containing samples to the surfaces of gels, most frequently slab gels used for protein or nucleic acid separations. In each case, these sample wells were designed to concentrate macromolecules in the sample into a thin starting zone prior to their migration through the resolving gel. The following references describe the use of devices placed against a gel to form wells: U.S. Pat. No. 5,304,292 describes the use of pieces of compressible gasket to form well walls at the top of a slab where the ends of the pieces touch the top edge of the slab. U.S. Pat. No. 5,164,065 describes a shark's tooth comb used in combination with DNA sequencing gels.
Several references describe automated devices for creating gradients of polymerizable monomers. Such systems have been used for making porosity gradient gels used in molecular weight separations of proteins. Altland et al. (Altland, K. and Altland, A. Pouring reproducible gradients in gels under computer control, Clin. Chem. 30(12 Pt 1):2098-2103, 1984) shows the use of such systems for creating the gradients of titratable monomers used in the creation of IPG gels. U.S. Pat. No. 4,169,036 describes a system for loading slab-gel holders for electrophoresis separation. U.S. Pat. No. 4,594,064 discloses an automated apparatus for producing gradient gels. Hence, use of a computer-controlled gradient maker in manufacturing IPG and other gels is known in the art.
One alternative method of running IPG strips in an IsomorpH device is disclosed in Disclosure Document No. 342751 (Anderson, N. L., entitled "Vertical Method for Running IPG Gel Strips"). The disclosed device uses sample wells pressed against the gel surface, but otherwise completely closed, so that the assembly could be rotated into a vertical orientation, thus allowing closer packing of gels and a greater gel capacity in a small instrument footprint. Additional methods are disclosed in Disclosure Document No's. 393753 (Anderson, N. L., Goodman, Jack, and Anderson, N. G., entitled "Gel Strips for Protein Separation") and 412899 (Anderson, N. L., Goodman, Jack, and Anderson, N. G., entitled "Automated System for Two-Dimensional Electrophoresis").
Systems for making non-planar slab gels are also known in the art and are disclosed in the following references: U.S. Pat. No. 5,074,981 discloses a substitute for submarine gels using an agarose block that is thicker at the ends and hangs into buffer reservoirs. U.S. Pat. No. 5,275,710 discloses lane-shaped gels formed in a plate and gel-filled holes extending down from the plate into buffer reservoirs. These gel systems, however, do not provide a gel which can be given a cross-section that is optimal for producing high-resolution protein separation. Furthermore, these systems cannot incorporate varying cross-sections along the length of a gel as required.
SDS Slab Gel Electrophoresis
Charged detergents such as sodium dodecyl sulfate (SDS) can bind strongly to protein molecules and "unfold" them into semi-rigid rods whose lengths are proportional to the length of the polypeptide chain, and hence approximately proportional to molecular weight. A protein complexed with such a detergent is itself highly charged (because of the charges of the bound detergent molecules), and this charge causes the protein-detergent complex to move in an applied electric field. Furthermore, the total charge also is approximately proportional to molecular weight (since the detergent's charge vastly exceeds the protein's own intrinsic charge), and hence the charge per unit length of a protein-SDS complex is essentially independent of molecular weight. This feature gives protein-SDS complexes essentially equal electrophoretic mobility in a non-restrictive medium. If the migration occurs in a sieving medium, such as a polyacrylamide gel, however, large (long) molecules will be retarded compared to small (short) molecules, and a separation based approximately on molecular weight will be achieved. This is the principle of SDS electrophoresis as applied commonly to the analytical separation of proteins.
An important application of SDS electrophoresis involves the use of a slab-shaped electrophoresis gel as the second dimension of a two-dimensional procedure. The gel strip or cylinder in which the protein sample has been resolved by isoelectric focusing is placed along the slab gel edge and the molecules it contains are separated in the slab, perpendicular to the prior separation, to yield a two-dimensional (2-D) separation. Fortunately, the two parameters on which this 2-D separation is based, namely isoelectric point and mass, are almost completely unrelated. This means that the theoretical resolution of the 2-D system is the product of the resolutions of each of the constituent methods, which is in the range of 150 molecular species for both IEF and SDS electrophoresis. This gives a theoretical resolution for the complete system of 22,500 proteins, which accounts for the intense interest in this method. In practice, as many as 5,000 proteins have been resolved experimentally. The present invention is aimed primarily at the 2-D application, and includes means for automating the second dimension SDS separation of a 2-D process to afford higher throughput, resolution and speed.
It is current practice to mold electrophoresis slab gels between two flat glass plates, and then to load the sample and run the slab gel still between the same glass plates. The gel is molded by introducing a dissolved mixture of polymerizable monomers, catalyst and initiator into the cavity defined by the plates and spacers or gaskets sealing three sides. Polymerization of the monomers then produces the desired gel media. This process is typically carried out in a laboratory setting, in which a single individual prepares, loads and runs the gel. A gasket or form comprising the bottom of the molding cavity is removed after gel polymerization in order to allow current to pass through two opposite edges of the gel slab: one of these edges represents the open (top) surface of the gel cavity, and the other is formed against its removable bottom. Typically, the gel is removed from the cassette defined by the glass plates after the electrophoresis separation has taken place, for the purposes of staining, autoradiography, etc., required for detection of resolved macromolecules such as proteins.
The concentrations of polyacrylamide gels used in electrophoresis are stated generally in terms of %T (the total percentage of acrylamide in the gel by weight) and %C (the proportion of the total acrylamide that is accounted for by the crosslinker used). N,N'-methylenebisacrylamide ("bis") is typically used as crosslinker. Typical gels used to resolve proteins range from 8% T to 24% T, a single gel often incorporating a gradient in order to resolve a broad range of protein molecular masses.
In most conventional systems used for SDS electrophoresis, use is made of the stacking phenomenon first employed in this context by Laemmli, U. K. (1970) Nature 227, 680. In a stacking system, an additional gel phase of high porosity is interposed between the separating gel and the sample. The two gels initially contain a different mobile ion from the ion source (typically a liquid buffer reservoir) above them: the gels contain chloride (a high mobility ion) and the buffer reservoir contains glycine (a lower mobility ion, whose mobility is pH dependent). All phases contain Tris as the low-mobility, pH determining other buffer component and positive counter-ion. Negatively charged protein-SDS complexes present in the sample are electrophoresed first through the stacking gel at its pH of approximately 6.8, where the complexes have the same mobility as the boundary between the leading (Cl-) and trailing (glycine-) ions. The proteins are thus stacked into a very thin zone "sandwiched" between Cl- and glycine-zones. As this stacking boundary reaches the top of the separating gel the proteins become unstacked because, at the higher separating gel pH (8.6), the protein-SDS complexes have a lower mobility. Thus, in the separating gel, the proteins fall behind the stacking front and are separated from one another according to size as they migrate through the sieving environment of the lower porosity (higher %T acrylamide) separating gel. In this environment, proteins are resolved on the basis of mass.
Pre-made slab gels have been available commercially for many years (e.g., from Integrated Separation Systems). These gels are prepared in glass cassettes much as would be made in the user's laboratory, and shipped from a factory to the user. More recently, methods have been devised for manufacture of both slab gels in plastic cassettes (thereby decreasing the weight and fragility of the cassettes) and slab gels bonded to a plastic backing (e.g., bonded to a Gelbond.RTM. Mylar.RTM. sheet or to a suitably derivatized glass plate). To date, all commercially-prepared gels are either enclosed in a cassette or bonded to a plastic sheet on one surface (the other being covered by a removable plastic membrane). Furthermore, all commercially-prepared gels have a planar geometry.
Current practice in running slab gels involves one of two methods. A gel in a cassette is typically mounted on a suitable electrophoresis apparatus, so that one edge of the gel contacts a first buffer reservoir containing an electrode (typically a platinum wire) and the opposite gel edge contacts a second reservoir with a second electrode, steps being taken so that the current passing between the electrodes is confined to run mainly or exclusively through the gel. Such apparatus may be "vertical" in that the gel's upper edge is in contact with an upper buffer reservoir and the lower edge is in contact with a lower reservoir, or the gel may be rotated 90.degree. about an axis perpendicular to its plane, so that the gel runs horizontally between a left and right buffer reservoir, as is disclosed in U.S. Pat. No. 4,088,561 (e.g., "DALT" electrophoresis tank). Various configurations have been devised in order to make these connections electrically, and to simultaneously prevent liquid leakage from one reservoir to the other (around the gel).
When used as part of a typical 2-D procedure, an IEF gel is applied along one exposed edge of such a slab gel and the proteins it contains migrate into the gel under the influence of an applied electric field. The IEF gel may be equilibrated with solutions containing SDS, buffer and thiol reducing agents prior to placement on the SDS gel, in order to ensure that the proteins the IEF gel contains are prepared to begin migrating under optimal conditions, or else this equilibration may be performed in situ by surrounding the gel with a solution or gel containing these components after it has been placed in position along the slab's edge.
A slab gel affixed to a Gelbond.RTM. sheet is typically run in a horizontal position, lying flat on a horizontal cooling plate with the Gelbond.RTM. sheet down and the unbonded surface up. Electrode wicks communicating with liquid buffer reservoirs, or bars of buffer-containing gel, are placed on opposite edges of the slab to make electrical connections for the run, and samples are generally applied onto the top surface of the slab (as in the instructions for the Pharmacia ExcelGels).
It is current practice to detect proteins in 2-D gels either by staining the gels or by exposing the gels to a radiosensitive film or plate (in the case of radioactively labeled proteins). Staining methods include dye-binding (e.g., Coomassie Brilliant Blue), silver stains (in which silver grains are formed in protein-containing zones), negative stains in which, for example, SDS is precipitated by Zn ions in regions where protein is absent, or the proteins may be fluorescently labeled. In each case, images of separated protein spot patterns can be acquired by scanners, and this data reduced to provide positional and quantitative information on sample protein composition through the action of suitable computer software.
Additional methods are disclosed in Disclosure Document No's. 393754 (Anderson, N. L., Goodman, Jack, and Anderson, N. G., entitled "Apparatus and Methods for Casting and Running Electrophoresis Slab Gels") and 412899 (Anderson, N. L., Goodman, Jack, and Anderson, N. G., entitled "Automated System for Two-Dimensional Electrophoresis").
Sample Preparation
Protein samples to be analyzed using 2-D electrophoresis are typically solubilized in an aqueous, denaturing solution such as 9M urea, 2% NP-40 (a non-ionic detergent), 2% of a pH 8-10.5 ampholyte mixture and 1% dithiothreitol (DTT). The urea and NP-40 serve to dissociate complexes of proteins with other proteins and with DNA, RNA, etc. The ampholyte mixture serves to establish a high pH (.about.9) outside the range where most proteolytic enzymes are active, thus preventing modification of the sample proteins by such enzymes in the sample, and also complexes with DNA present in the nuclei of sample cells, allowing DNA-binding proteins to be released while preventing the DNA from swelling into a viscous gel that interferes with IEF separation. The purpose of the DTT is to reduce disulfide bonds present in the sample proteins, thus allowing them to be unfolded and assume an open structure optimal for separation by denaturing IEF. Samples of tissues, for example, are solubilized by rapid homogenization in the solubilizing solution, after which the sample is centrifuged to pellet insoluble material and DNA, and the supernatant collected for application to the IEF gel.
Because of the likelihood that protein cysteine residues will be come oxidized to cysteic acid or recombine and thus stabilize refolded, not fully denatured protein structures during the run, it is desirable to chemically derivatize the cysteines before analysis. This is typically accomplished by alkylation to yield a less reactive cysteine derivative.
Use of 2-D Electrophoresis
Two-dimensional electrophoresis is widely used to separate from hundreds to thousands of proteins in a single analysis, in order to visualize and quantitate the protein composition of biological samples such as blood plasma, tissues, cultured cells, etc. The technique was introduced in 1975 by O'Farrell, and has been used since then in various forms in many laboratories.
The gel systems known in the art or referenced above, however, do not provide an integrated, fully automated, high-throughput system for two-dimensional electrophoresis of proteins. Moreover, current IPG and slab gel systems are not fully automated, wherein all operations including gel casting, processing, sample loading, running and final disposition are carried out by an integrated, fully automated system. Current gel systems cannot be fully controlled by a computer and cannot systematically vary gel, process, sample load and run parameters, provide positive sample identification, and cannot collect process data with the object of optimizing the reproducibility and resolution of the protein separations.